1. Technology Field
Embodiments of the present invention generally relate to x-ray generating devices. More specifically, example embodiments relate to an electron shield configured to intercept and absorb backscattered electrons and having a construction that reduces heat-related damage.
2. The Related Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, and then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device. The x-ray tube generally comprises a vacuum enclosure, a cathode, and an anode. The cathode, having a filament for emitting electrons, is disposed within the vacuum enclosure, as is the anode that is oriented to receive the electrons emitted by the cathode.
The vacuum enclosure may be composed of metal such as copper, glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. The entire outer housing is typically covered with a shielding layer (composed of, for example, lead or similar x-ray attenuating material) for preventing the escape of x-rays produced within the vacuum enclosure. In addition a cooling medium, such as a dielectric oil or similar coolant, can be disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure. Depending on the configuration, heat can be removed from the coolant by circulating it to an external heat exchanger via a pump and fluid conduits.
In operation, an electric current is supplied to the cathode filament, causing it to emit a stream of electrons by thermionic emission. In anode end grounded (AEG) x-ray tubes, a high negative electric potential is placed on the cathode while the anode is electrically grounded. This causes the electron stream to gain kinetic energy and accelerate toward a target surface disposed on the anode. Upon impingement at the target surface, some of the resulting kinetic energy is converted to electromagnetic radiation of very high frequency, i.e., x-rays.
The characteristics of the x-rays produced depend in part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (“Z numbers”), such as tungsten or TZM (an alloy of titanium, zirconium, and molybdenum) are typically employed. The resulting x-rays can be collimated so that they exit the x-ray device through predetermined regions of the vacuum enclosure and outer housing for entry into the x-ray subject, such as a medical patient.
One challenge encountered with the operation of x-ray tubes relates to backscattered electrons, i.e., electrons that rebound from the target surface along unintended paths in the vacuum enclosure. Depending on the environment, upwards of thirty percent of the electrons traveling from the cathode to the anode hit and bounce from the point of impingement. These rebounding, backscattered electrons can impact areas of the x-ray tube where such electron impact is not desired. These impacts result in the generation of excess heat that can damage the impacted component.
To minimize the effects of backscattered electrons, a backscatter electron collection device, sometimes referred to as an “aperture” or “aperture shield,” can be included in x-ray tubes. Such a device can be interposed between the electron emitting filament of the cathode and the anode target surface. The device can include an aperture through which the primary electrons can pass from the filament toward impingement on the target surface. In addition, the collection device is configured to intercept most of the electrons that subsequently rebound from the target. By collecting at least a portion of the backscattered electrons, the collection device acts as a shield and prevents their impingement on less desirable portions of the x-ray tube.
Although this collection of backscattered electrons at the electron collection device protects other portions of the x-ray tube, it nonetheless can give rise to problems in the collection device itself. In particular, the energy associated with the backscattered electrons heats the aperture causing it to expand. At a certain input power level the amount of expansion exceeds the aperture material's yield point causing plastic deformation due to thermal stresses. Repeated heating cycles associated with repeated x-ray exposures leads to aperture failure due to cracking and delamination. In addition, repeated contraction and expansion increases the number and rate at which particles are detached from the electron shield, known as “particulation rates,” which results in tube arcing.
Failure of the electron shield in the manner described above is detrimental to tube performance. For example, the electron shield might define a portion of the vacuum envelope in which critical tube components, such as the cathode and anode, are housed. Upon failure of the electron shield, the vacuum can be compromised thereby rendering the x-ray tube useless, requiring its replacement often at significant cost. At a minimum the thermal-induced damage reduces tube operating life.
Attempts to completely solve some of the above problems have not been entirely successful and/or desirable. For example, the use of expensive copper alloys such as Glidcop™ have been used to reduce plastic deformation at the aperture. However, such materials are expensive and typically cannot be operated at higher operating powers, e.g., lower than approximately 72 kW. Other solutions might involve providing an electron shield with a larger diameter aperture. However, this approach reduces the overall number of backscattered electrons that are captured, allowing a greater percentage to rebound back to the surface of the anode and thereby affecting image quality.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.